Forensic Science

10 Progress in Firearm Residue Detection EDGARS RUDZITIS Illinois B u r e a u of Identification, Joliet, Ill. 60432 MORRIS A. WAHLGREN

Downloaded by UNIV OF PITTSBURGH on June 22, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0013.ch010

A r g o n n e N a t i o n a l L a b o r a t o r y , A r g o n n e , Ill. 60439

The concept of Neutron A c t i v a t i o n A n a l y s i s (NAA) of barium and antimony present in primers as a means of f i r e a r m residue d e t e c t i o n is now ten years o l d and it appears appropriate to point out the s a l i e n t p o i n t s of the development of the technique. The o r i g i n a l work (1) and l a t e r i n v e s t i g a t i o n by high speed photography and autoradiography (2) conclude that Ba from primer and b u l l e t are deposited on the back of the firing hand as d i s c r e t e p a r t i c u l a t e matter. The obvious problem f o r the detec- t i o n of firearms residue on s k i n i s the e f f i c i e n c y of c o l l e c t i o n both from the standpoint of convenience and q u a n t i t a t i o n . A f t e r e x p l o r i n g a v a r i e t y of methods, p a r a f f i n cast was suggested as the best (3) while another i n v e s t i g a t i o n suggests a complete hand r i n s e (4) as the most q u a n t i t a t i v e technique. C o l l o d i o n lifts have been reported (5) and a comparison between c o l l o d i o n lifts and filter paper swabs concludes (2) that both techniques are about e q u a l l y efficient. Sampling by moistened c o t t o n swab has been a l s o suggested (5,6) as a convenient technique. While there has been considerable experimentation with sampling techniques, there have been few attempts to improve the a n a l y t i c a l procedure. The o r i g i n a l separation method (1) has undergone only minor changes (6-8). There have been two attempts to optimize the radiochemical procedure and improve the accuracy by i n t r o d u c i n g Ba and Sb as t r a c e r s (9,10) and by r e - e x p l o r a t i o n of Sb radiochemistry (10). A p u r e l y instrumental method has been reported (11) at the expense of excluding the determination of barium. The b i v a r i a t e - l o g - n o r m a l a n a l y s i s of data c o l l e c t e d by Guinn and co-workers appears to be the only comprehensive statistical treatment of f i r e a r m residue d e t e c t i o n by NAA (11). Suspects' handswabs were i n t e r p r e t e d i n terms of accumulated firing t e s t data and handblanks c o l l e c t e d from i n d i v i d u a l s of d i f f e r e n t occu- p a t i o n a l backgrounds. A somewhat more e m p i r i c a l i n t e r p r e t a t i o n of the same data is a l s o reported (12). A d d i t i o n a l data from smaller s c a l e c o l l e c t i o n of handblanks have been published r e c e n t l y (13,14).

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In Forensic Science; Davies, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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The NAA method f o r the determination of f i r e a r m discharge residue has been g e n e r a l l y accepted, but a p p l i c a t i o n s have been l i m i t e d to j u s t a few l a b o r a t o r i e s . In the process of e s t a b l i s h ing NAA c a p a b i l i t y f o r the State of I l l i n o i s crime l a b o r a t o r i e s we re-examined the standard techniques (10). In the course of our work i t became c l e a r that p o s t - i r r a d i a t i o n i s the cause of s e v e r a l c o n s t r a i n t s which have discouraged a more widespread use of NAA. The inherent time l i m i t a t i o n due to the 87 min. h a l f - l i f e of B a n e c e s s i t a t e s f a s t manipulations of r a d i o a c t i v e s o l u t i o n s which i n turn r e q u i r e s an experienced radiochemist. In a d d i t i o n to an ever present danger of overexposure and contamination, t y p i c a l l y only a dozen samples can be i r r a d i a t e d per batch, which makes the method q u i t e expensive. The developed s t a t i s t i c a l b i v a r i a t e - n o r m a l a n a l y s i s (11) i s convenient f o r r o u t i n e a p p l i c a t i o n s . With t h i s i n mind, a method was developed which: a) e l i m i n a t e s p o s t - i r r a d i a t i o n r a d i o c h e m i s t r y and thus maximizes time f o r a n a l y s i s ; b) accommodates over 130 samples per i r r a d i a t i o n capsule ( r a b b i t ) ; c) does not r e q u i r e a c o l l e c t i o n of o c c u p a t i o n a l handblanks; and d) u t i l i z e s a s i m p l i f i e d s t a t i s t i c a l concept based on n a t u r a l antimony and barium l e v e l s on hands f o r the i n t e r p r e t a t i o n of data. The d e t a i l e d procedure w i l l be published elsewhere (15). B r i e f l y , a cotton swab technique i s employed; a f t e r swabbing Sb and Ba are f i x e d on the c o t t o n of the swab by moistening i t w i t h an aqueous s o l u t i o n of thioacetamide (TAA) and then w i t h d i l u t e s u l f u r i c a c i d . The cotton of the swab i s then s t r i p p e d and s o l u b l e i n t e r f e r r i n g m a t e r i a l s , n o t a b l y NaCl, are removed by l e a c h i n g with 2N HgSO^ and methanol. The t r e a t e d c o t t o n i s compacted i n t o a small p i e c e of %" polyethylene tubing and i r r a d i a t e d a t a neutron f l u x of 5x10 N/cm /sec. Best r e s u l t s were obtained by double i r r a d i a t i o n : 40 min. with immediate count f o r barium and 5 h r s with 2 day c o o l i n g f o r antimony. The photo peaks of Ba and S b were determined u t i l i z i n g a Ge-Li d e t e c t o r with a computerized pulseheight a n a l y s i s system. With the a v a i l a b i l i t y of some 50 s e t s of handblanks ( e n v i r onmental-natural l e v e l s of Ba and Sb on hands), f i r i n g t e s t s and c a l i b r a t i o n s , we considered a d i f f e r e n t concept f o r the i n t e r p r e t a t i o n of the r e s u l t s . The e v a l u a t i o n c o n s i s t e d of two s t e p s : 1) e s t a b l i s h i n g t h a t the Ba and Sb v a l u e s of handblanks of the accumulated p o p u l a t i o n sample followed a normal (Gaussian) d i s t r i b u t i o n as s t a t i s t i c a l l y approximated by the t - D i s t r i b u t i o n , and 2) u t i l i z a t i o n of r e l a t i v e l y simple s t a t i s t i c a l formalism f o r the c a l c u l a t i o n of the p r o b a b i l i t y t h a t the amount of Ba and Sb found on a given swab belongs to the e s t a b l i s h e d handblank populat i o n . (An appendix at the end of the paper may be u s e f u l to readers not normally u t i l i z i n g s t a t i s t i c s ) .


1 3 9

>

12

1 3 9

1 2 2


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Calibration In order to t e s t the r e l i a b i l i t y of the chemical and i r r a d i a t i o n procedure, d e t e c t i o n , and data r e d u c t i o n d e s c r i b e d i n r e f e r e n c e (15), each i r r a d i a t i o n batch had s e v e r a l s e t s of chemical standards. In Table 1, " i n " represents the amounts i n nanograms, ng, of Sb and Ba standards deposited on swabs, the mean of 12 d e t e r minations each i s i n the "found" column and the standard d e v i a t i o n , " s t d . dev.", i s given as a percentage of the "found" v a l u e . The average value i n terms of CPM (counts per minute) per ng c a l c u l a t ed from the three concentrations (5,10 and 20ng) y i e l d e d 98 CPM ± 27% f o r Ba and was used as a b a s i s to c a l c u l a t e "found" v a l u e s . TABLE I CALIBRATION DATA (nanograms) Sb in

found

5 10 20

5 8 22

Ba std.dev. ( % )

a

in

27 18 32

50 100 200

std.dev. expressed i n % of the

found

49 101 270

Std.dev.(%)

a

22 14 25

"found" v a l u e .

G r a p h i c a l e x t r a p o l a t i o n y i e l d e d l i n e a r p l o t s through the o r i g i n w i t h i n a corresponding one standard d e v i a t i o n range f o r each concentration. (Previous t r a c e r experiments have shown a v i r t u a l 100% r e t e n t i o n of *Sb and B a on c o t t o n , (10)) . Considering the involved path from sampling through data reduct i o n , the method appears to be s u f f i c i e n t l y r e l i a b l e t o warrant a s t a t i s t i c a l analysis. 121

1 3 3

Handblank Values Table 2 represents the mean values and standard d e v i a t i o n s i n percent of the mean value of the f o u r separate hand swabs, r i g h t back, (RB), r i g h t palm, (RP), l e f t back (LB), l e f t palm (LP), c o l l e c t e d from 30 persons. The hands of crime l a b t e c h n i c i a n s and p o l i c e o f f i c e r s were swabbed "as i s " without any pretreatment. While there were o c c a s i o n a l higher readings on palms, the found values were a l l represented by the means and standard d e v i a t i o n s . In only one instance were high Ba values ( l e s s than 1:333 p r o b a b i l i t y being ^normal ') encountered on a l l four s u r f a c e s . when q u e s t i o n 1



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ed, the i n d i v i d u a l remembered having used a carbon-based s o l i d l u b r i c a n t the night before, which was d i f f i c u l t t o remove from the hands and t h e r e f o r e q u i t e l i k e l y was the o r i g i n of the high Ba determination; n e v e r t h e l e s s , these values weiealso included i n the c a l c u l a t i o n of the means. As can be seen from the t a b l e , the mean values of the handblanks d i d not d i f f e r s i g n i f i c a n t l y i n terms of standard d e v i a t i o n s and i t was convenient to lump them as the t o t a l hand averages of 0.4 and 7 ng of Sb and Ba r e s p e c t i v e l y , w i t h an approximated standard d e v i a t i o n of ± 100%, i . e . , ± 0.4 and t 7 ng r e s p e c t i v e l y . While our values are lower than those encountered by previous workers,the r e l a t i v e r e l a t i o n s h i p s are e s s e n t i a l l y unchanged and the data are s e l f - c o n s i s t e n t . TABLE I I HANDBLANK VALUES (nanograms) Antimony

Barium

RB

RP

LB

LP

RB

RP

LB

LP

mean (ng)

0.4

0.5

0.3

0.4

6

8

5

10

std.dev.(%)

81

116

87

110

116

87

128

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t - D i s t r i b u t i o n of Sb and Ba Values of

Handblanks

There i s no a p r i o r i reason to doubt that the C e n t r a l L i m i t Theorem, and consequently the normal d i s t r i b u t i o n concept, app l i e s to t r a c e element d i s t r i b u t i o n , i n c l u d i n g Sb and Ba on hands i n a human population, because these c o n c e n t r a t i o n s are a f f e c t e d by such random v a r i a b l e s as l o c a t i o n , d i e t , metabolism, and so on. However, s i n c e enough data were at hand (some 120 samples per element), i t was of i n t e r e s t to t e s t the normal d i s t r i b u t i o n experimentally by examination of the t - D i s t r i b u t i o n . The probab i l i t y d e n s i t y p l o t s of 0.2 and 3 ng increments f o r Sb and Ba, r e s p e c t i v e l y , had s i m i l a r appearances. The a c t u a l d i s t r i b u t i o n t e s t was c a r r i e d out f o r Sb only because of b e t t e r data due t o the more convenient h a l f l i f e of S b . After normalization, a "one t a i l " t e s t was c a r r i e d out. Normalization c o n s i s t e d of s h i f t i n g the s c a l e one standard d e v i a t i o n to the l e f t so that the o r i g i n and maximum (mean) were superimposed. A l s o , the v e r t i c a l s c a l e was s h i f t e d i n order to superimpose the t h e o r e t i c a l and experimental p r o b a b i l i t y d e n s i t i e s at the o r i g i n . The t h e o r e t i c a l values were c a l c u l a t e d from the expression f ( Z ) = (2TT) ^ / e ^ d z where z= (x.-x)/s=x/s ( f o r d e f i n i 1 2 2

z 2


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t i o n s see Appendix) s i n c e x=o as a r e s u l t o f the s h i f t . The t h e o r e t i c a l and experimental values are compared i n Table 3 as a f u n c t i o n of weight increments of antimony. As can be seen, the agreement i s as good as can be expected f o r an average o f 13 samples per weight increment. The experimental values show the t a i l expected f o r a t - D i s t r i b u t i o n . Therefore, the a p p l i c a t i o n of t - t e s t s to determine the p r o b a b i l i t y o f a given antimony amount of a hand swab being n a t u r a l background i s j u s t i f i e d . By analogy, the same c o n c l u s i o n may be made f o r barium l e v e l s . TABLE 3 EXPERIMENTAL t-DISTRIBUTION VS. NORMAL DISTRIBUTION FOR Sb HANDBLANKS x (ng Sb)

Theoretical

Experimental

42 32 19 9 3 1 0

42 32 15 14 7 5 4 0 1

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

Comparison of Handblanks and F i r i n g T e s t s . T h i r t e e n known one hand-one shot f i r i n g s of both automatics and r e v o l v e r s o f v a r i o u s c a l i b e r s were s e l e c t e d . The means and ranges o f antimony and barium values f o r the f i r i n g hand a r e presented i n Table 4. TABLE 4 FIRING TEST DATA FIRING HAND BACK (FHB) Sb

a

FIRING HAND PALM (FHP)

Ba

Sb

Ba

6.6

68

0.4-26

0-223

mean:

10.0

144

range:

2.5-27

27-660

a A l l u n i t s are ng.


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I f the means are taken as a c h a r a c t e r i s t i c t e s t f i r i n g , i t i s obvious from the range that standard d e v i a t i o n has l i t t l e meaning f o r comparison with handblanks - one of the d i f f i c u l t i e s encountered i previous i n v e s t i g a t i o n s when s t a t i s t i c s were based on t e s t f i r i n g s (11, 12). An i n t e r e s t i n g note i s that the f i r i n g hand back, FHB, has roughly twice the amount of Sb and Ba as the palm, FHP, which i n turn shows about ten times the amounts of a t y p i c a l handblank. In order to q u a n t a t i v e l y compare the means of the f i r i n g t e s t s and those of handblanks, we w i l l use the so c a l l e d N u l l Hypothesis i n conjunction with the t - T e s t . The N u l l Hypothesis d i v i d e s the expected r e s u l t s i n t o two c l a s s e s , the acceptance (true) c l a s s and the r e j e c t i o n ( f a l s e ) c l a s s , i n such a way t h a t the p r o b a b i l i t y of the r e j e c t i o n c l a s s when the N u l l Hypothesis i s true i s equal to some small preassigned value c a l l e d the s i g n i f i c a n c e l e v e l . In t h i s i n s t a n c e , the hypothesis w i l l be that the f i r i n g t e s t data represent handblanks. (This i s not an unreasonable assumption, because whenever a handswab i s taken i n a case i t has to be assumed that the s u b j e c t has a normal handblank u n t i l proved o t h e r w i s e ) . In other words, because of p o t e n t i a l l y s e r i o u s i m p l i c a t i o n s , every attempt should be made to avoid the e r r o r of r e j e c t i n g the N u l l Hypothesis (conc l u s i o n of abnormally l a r g e Sb and Ba amounts) when a c t u a l l y the hypothesis i s true (normal handblanks). U t i l i z i n g the t - D i s t r i b u t i o n t a b l e s of Bulmer s t e x t (16) (see Appendix), the most conservative l i m i t was chosen namely a 1:2000 p r o b a b i l i t y of r e j e c t i n g the N u l l Hypothesis when a c t u a l l y t r u e ( i . e . , r e j e c t ing the assumption of normal handblanks). The degree of freedom was taken as 120 ( a c t u a l l y 119) because, as Table 2 shows, the combination of a l l four data f o r a handblank i s j u s t i f i e d . Table 5 i l l u s t r a t e s the t - t e s t by comparing the mean Sb and Ba values of f i r i n g t e s t s and handblanks. The wide disagreement between the c a l c u l a t e d and t h e o r e t i c a l i s not s u r p r i s i n g i f one considers that i n a normal d i s t r i b u t i o n there i s a 1:333 probab i l i t y that a value i s o u t s i d e three standard d e v i a t i o n s , whereas i n t h i s example even f i r i n g hand palm values are approximately 10 standard d e v i a t i o n s away i n terms of mean and standard d e v i a t i o n of the handblanks. The i m p r o b a b i l i t y of matching the data of handblanks and f i r i n g t e s t s i s made even more astronomical by the m u l t i p l i c a t i o n r u l e of s t a t i s t i c s because both elements, Sb and Ba, have to be considered. A p o s i t i v e c o r r e l a t i o n between back and palm values c e r t a i n l y should have an added s t a t i s t i c a l s i g n i f i c a n c e as to the abnormally high antimony and barium values of the f i r i n g t e s t s .


n

1

f

CONCLUSIONS I t should be reemphasized that the complete i n c o m p a t i b i l i t y of f i r i n g t e s t and handblank values are based s t r i c t l y on the s t a t i s tical treatment of the two sets of data accumulated so f a r . A more general a p p l i c a b i l i t y w i l l h o p e f u l l y emerge as more hand-


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blanks are accumulated. I t can be r e a l i s t i c a l l y argued, though, that environmental handblanks of sharpshooters, employees of fireworks f a c t o r i e s , or s c i e n t i s t engaged i n research of Sb-Ba a l l o y s may be s i g n i f i c a n t l y h i g h e r . While the above s t a t i s t i c a l reasoning may not yet be a p p l i e d w i t h i n the present j u d i c i a l system, i t at l e a s t provides a backup f o r reaching d e c i s i o n s . A f t e r a l l , the c o r r e c t n e s s of the judgment of an expert witness i s a l s o subject to the laws of statistics. APPENDIX Sample, x i s a s i n g l e measurement of a datum from a population. ** ** Mean and average, x, can be used i n t e r changeably. The standard d e v i a t i o n , s, i s a u s e f u l parameter f o r a reasonably l a r g e p o p u l a t i o n ; n, i s , i n e f f e c t , an absolute average d e v i a t i o n from the mean. The degree of freedom i s n-1 ( i . e . , with n=l no s t a t i s t i c s i s p o s s i b l e ) . The p r o b a b i l i t y d e n s i t y i s the percentage of samples w i t h i n a given range. Normal and t - D i s t r i b u t i o n s : the Normal D i s t r i b u t i o n i s r e p r e sented by the well-known bell-shaped curve, where the maximum i s represented by the mean, x, and the standard d e v i a t i o n , s, i s the width at the i n f l e c t i o n p o i n t . The b a s i s of i t s a p p l i c a t i o n to many n a t u r a l phenomena i s the C e n t r a l - L i m i t Theorem, which s t a t e s that the sum of a l a r g e number of independent v a r i a b l e s w i l l be approximately normally d i s t r i b u t e d r e g a r d l e s s of t h e i r i n d i v i d u a l d i s t r i b u t i o n s . The theory of the normal d i s t r i b u t i o n was d e v e l oped from a l a r g e number of samples, which, of course, can not be s t r i c t l y a p p l i c a b l e i n most p r a c t i c a l cases. T h i s was recognized e a r l y by an I r i s h chemist W.S. Gosset (17) who formed the b a s i s of of the "small sample theory" - the so c a l l e d t - D i s t r i b u t i o n - i n c l u d i n g - t - T e s t . The theory, i n e f f e c t , s t a t e s that the t - D i s t r i but ion has the same shape asthe normal d i s t r i b u t i o n except that the curve i s f l a t t e r and has a longer t a i l . The t-Test,which i s an estimate of the p r o b a b i l i t y by which a given sample f a l l s i n t o an e s t a b l i s h e d t - D i s t r i b u t i o n , i s represented by the formula t = ( x - x / s ) n 2 , where x,^ i s the value of a given sample and x, s, 9

n

x

i

and n are the parameters of a p r e v i o u s l y e s t a b l i s h e d t - D i s t r i b u t i o n . The c a l c u l a t e d t-value i s compared with tabulated t h e o r e t i c a l values and the p r o b a b i l i t y of f i t (or m i s f i t ) i s thus d e t e r mined . The "One t a i l " , i n which only one h a l f of the curve i s t e s t e d , i s i n d i c a t e d when below-average values are not of i n t e r e s t or when the s e n s i t i v i t y i s l i m i t e d (as i n very low l e v e l antimony, barium d e t e r m i n a t i o n s ) . The "small sample s t a t i s t i c s " i s widely used and has obvious a p p l i c a t i o n s i n c r i m i n a l i s t i c s . A number of s u i t a b l e t e x t s are a v a i l a b l e (16,18).


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TABLE 5 ILLUSTRATION OF THE t-TEST Sb x

(FHB)

10.0 ng

128 ng

x (FHP)

6.3 ng

68 ng

X

0.4 ng

7 ng

s

0.4 ng

7 ng

±

±


Ba

t (FHB)

190

143

Tabulated t-value ( a p p l i e s both to Sb and Ba) f o r m=120 and at a s i g n i f i c a n c e l e v e l of 0.0005 i s 3.4; the corresponding t-value f o r a s i g n i f i c a n c e l e v e l 0.01 i s 2.4. FHB=firing hand back, FHP=firing hand palm.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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FORENSIC SCIENCE


Forensic Science

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